Art of aluminum etching



March 19, 1946. co s 2,396,685

ART OF ALUMINUM ETCHING Filed Nov. 7, 1940 3 Sheets-Sheet 1 INVENTORGeo/ e Z'Cbyyi/w J yaw-M 6m 1 m ATTO NEY 5.

March 19, 1946. COGGINS 2,396,685

ART OF ALUMINUM ETCHING Filed Nov. 7, 1940 5 Sheets-Sheet 2 INVENTORGeo/ye 11 00 am BY 1 a ATTIZIZW March 19, 1946.

G. F. COGGINS ART OF ALUMINUM ETCHING Filed NOV. 7, 1940 3 Sheets-Sheet5 OOOOO lNVENTOR Georye 17003 5115 M QQMqM ATTORNEYS Pasta Mar. 19, 1946ART OF ALUMINUM ETCHIN G George Frederick Coggins, New Bedford, Mass.,

assignor to Aerovox Corporation, New Bedl'ord, Mass, a corporation ofNew York Application November "I, 1940, Serial No. 364,625

Claims.

The present invention is particularly concerned with the etching ofaluminum plates or foils.

An object of the invention is to provide an etching process by whichaluminum foil or plates may be etched in large scale quantity productionwith uniformity within relatively close tolerances as to surface gainattained, all by simple procedure, with simple equipment and at lowcost.

The etching process while useful in preparing a roughened surface foraluminum foil or plates in a variety of other arts, has especial utilityin the preparation of electrodes for electrolytic condensers, in whichapplication great economy is attained in the electrode material requiredfor any given capacitance at any given voltage.

As conducive to a clear understanding of the invention, it is noted thatwhere the etching of aluminum plates or foils is practiced without theparticular controls of the present invention, the gain attained insurface area of foil may vary widely from section to section of one andthe same lot or batch of aluminum. The length of foil area needed toassure at least a given efiective surface area would have to beconsiderably greater than where the controls of the present inventionare utilized on the etching process, since the latter brings aboutagreatly reduced range of variation in the high surface gain attained.

The present invention results from my discovery that the efficacy anduniformity of the etching with hydrochloric acid for high surface gaindepends upon the control of a number of factors that heretofore were notsuspected to have any bearing upon the etching eflicacy, and would beexpected by those skilled in the art to have no bearing whatsoever uponetching eflicacy. Among these factors are an optimum temperature for anetching bath of any given concentration; the concentration of thecaustic solution with which the foil as it comes from the mill must betreated prior to etching; the concentration within the etching liquid ofthe aluminum salts that pass into solution as the etching proceeds, andthe iron content of the etching solution and of the sheet stock to beetched. Failure to control any of these factors results in reduction inthe etching gain attained, regardless how closely the other factorsabove-mentioned may be controlled.

While the desired uniformity of surface gain of the aluminum foil may beattained by controlling the various variables referred to, I have madeextensive tests to determine not only the conditions for uniformity ofsurface gain, but also those for such gain of optimum magnitude. Theresults of these tests are shown in the graphs on the drawings and areset forth in the specification.

By effecting the control in the manner hereinafter set forth, I havesucceeded in producing aluminum foil with substantially uniform gain insurface area in predetermined fixed ratio selected anywhere from four toten times the area of the plain foil and kept within tolerances of aboutplus or minus ten per cent. This result, while useful in otherrelations, spells tremendous economy in the manufacture of electrolyticcondensers. For a 450 volt working, 525 volt peak electrolytic condenserfor instance, an area of 1.5 square inches (and when desired, as littleas 0.8 square inch) per microfarad has been attained in the finishedcondenser.

In the accompanying drawings in which are diagrammatically shown thegraphs indicating the process in its general application and oneimportant specific application thereof,

Fig. 1 is a set of graphs indicating the variations in the gain attainedin capacitance per unit area, where thesubsequent etching process isperformed after pre-treatment of the foil with different concentrationsof caustic solution,

Fig. 2 is a set of graphs indicating the relationship of the capacitanceper unit area attained at various temperatures of given concentrationsof hydrochloric acid when treated for various lengths of time andindicating the existence of points of optimum working conditions foreach concentration used,

Fig. 3 is a pair of graphs indicating the effect upon the resultantcapacitance per unit of area of change, in concentration of the aluminumchloride produced in the etching bath,

Fig. 4 is a graph indicating the capacitance attained per unit area withvarying concentratibns of iron in the etching solution,

Fig. 5 is a diagrammatic view illustrating the pro-treating and etchingset up, and

Fig. 6 is a diagrammatic view illustratin the 2 I pared with thecapacitance with identical unetched foil subjected to like formingtreatment,

which yields a capacitance of but 0.18 microfarad per square inch.

Aluminum plates or foil The :foil or plate is preferably in rolls thatmust be continuous and free from patches or splices. When unwrapped thefoil must not stick so as to become torn or marred and should present nosubstantial camber. The surface should be substantially free fromlubricants or any such foreign material as is not readily removable inthe caustic treatment to which the foil is subjected prior to etching.

Where foil is used that has been annealed for about two days at about900 degrees F. and where the rate of cooling is slow and closelycontrolled the gain in area due to etching may be maintained at auniform value, up to in the'order of 40 per cent greater than whereunannealed foil is used. On the other hand, where the foil has beenannealed for a long time at high temperatures, and with uncontrolledcooling the efficacy of the etching again falls off materially from theoptimum value.

The foil should be in the order of 99.8, preferably 99.82 to 99.85 percent pure. In the less than two-tenth per cent impurity, there willordinarily be present small proportions of copper, silicon and iron. Thecopper content should be kept down to little more than a trace, the ironto about .08.l2 per cent and the silicon to about .06 per cent.

In practice a small test sample of each batch of foil delivered from thefoil manufacturer is subjected to the pre-treating and etching procedurehereinafter set forth. The test sample is per cent caustic solution at200 degrees F. for

one-half minute, the removal of oxide and oil is complete for allpractical purposes and thorough,

' while the attack by the caustic upon the metallic would be similar foretching baths of other conthen formed with a dielectric film and itscapacitance is measured to make sure that the foil is of character suchas to attain the desired gain in surface area. In practice but anoccasional batch of foil ordered from the manufacturer has to berejected.

Treatment prior to etching The set of graphs shown in Fig. lindicatesthe relation of the concentration of the caustic bath at a giventemperature to the capacitance per unit area attained as a result ofsubsequently etching in a definite solution of hydrochloric acid undervarious temperature conditions. This ca.- pacitance per unit area issubstantially a measure of the surface area attained by the etching andthe graphs are applicable therefore, to determine the control required,whether the desired gain in area is required for electrolytic condenseror for other purposes.

centrations.

Just why the concentration of the caustic treatment step affects theratio of surface gain in the subsequent etching step is not known orunderstood. Evidently, aside from the removal of oxide and oil as aboveset forth, this step results in some important modification, not visibleto the eye, in the surface structure of the aluminum, and it certainlylevels out to a considerable degree the variations in the surface of thealumi num as it is delivered from the rolling mill. Since therefore thecaustic bath does something more than to clean the foil, 1 designate thebath as a pre-treatment rather than merely as a cleaning bath.

It has been found that the accumulation of aluminum compounds in thecaustic solution due to the slight attack of the metal thereby has nodeleterious effect upon the eificacy of the caustic treatment. Desirablyhowever, the concentration of aluminum compounds is not allowed to reacha value so high as to come out of solution and to form a cake on thetank. With that one qualification, it is the concentration of thecaustic which alone is important at this stage of the process.

To guard against the considerable falling of! in efficacy of thesubsequent etching step as the caustic solution changes inconcentration, it is therefore important to adjust that concentration atfrequent intervals to maintain approximate constancy.

For practical purposes substantial constancy in concentration of causticmay be attained by testing and adjusting the caustic bath at intervals,desirably of about one-half hour. The adjustment is effected by theaddition of caustic or water to the bath as the case may be orpreferably by continuous addition of liquid caustic and water throughseparate faucets at the required rate. Such aluminum hydroxide as isformed by attack of the foil by the caustic reacts with the titratinghydrochloric acid in the same As will be seen from the graphs, the gainin surface area attained in the etching procedure rises with theconcentration of the caustic kept at uniform temperature in thepre-treating tank. Pie-treatment with a caustic concentration of two percent results in a better surface gain in the subsequent etchingoperation than with a lower concentration of caustic. With theparticular foil used no advantage is however gained by foil. Byimmersing the aluminum in the two way as does the sodium hydroxide, butdoes not contribute to the pre-treatment of the foil. For correctmeasurement of the caustic concentration a double titration is thereforeresorted to.

' One desirable titration method is the following:

To a given portion of the caustic tank solution add a predeterminedamountof normal hydrochloric acid solution. Add a drop of Metanil Yellowindicator and titrate with a normal solution of sodium hydroxide to ayellow color. Then add three drops of phenolphthalein indicator andtitrate with normal solution of sodium hydroxide to a pink color. Fromthe two end points a calculation of the aluminum and sodium hydroxideconcentration is readily made.

After pre-treatment has been completed the 'foil is washed with cold tapwater and then subjected to the etching which will now be set forth.

Etching When foil previously subjected to the caustic treatment abovedescribed, is etched in hydrochloric acid, the conditions governing thegain in surface area are pictured in the graph shown in Fig. 2. For agiven concentration of hydrochloric acid,- shown in the graph at theleft in the drawing as 2.44 N, there is a temperature of etching atwhich the optimum gain in superficial area is attained for any giventime of etching, the gain dropping off sharply when a temperature isused that is more than five degrees either lower or higher than suchoptimum. In substantially the five degree range to either side of theoptimum temperature, approximate maximum etching ratio is maintained.

As appears from Fig. 2, the temperature of the etching bath of 2.44 N toattain maximum surface gain in one and one-half minutes of immersion is155 degrees F. but if the temperature is lowered to 125 degrees F. thesurface gain attained is reduced by about fifty per cent.

As moreover appears from such graph, if the period during which theetching is conducted is increased, not only is there added gain insurface area as might be expected, but the temperature of the etchingbath at which optimum effectiveness of surface gain occurs is decreasedas shown. It is noted that if the temperature of the etching bath iswell above that for optimum results the reduced efiicacy is nearlyuniform regardless of the time during which the etching is conducted.

A family of curves similar to those shown at the left of Fig. 2 might beprepared for each concentration of the etching bath. For instance, as 2shown in the curve at the right of Fig. 2 where a 4 N solution ofhydrochloric acid is employed, the maxima of surface area gain aresomewhat higher than with the 2.44 N solution, and the temperatures oftreatment at which such maxima occur are lower than with the more dilutesolution.

Thus, depending upon the surface gain desired, etching solution would beselected of concentration, and the etching would be conducted for theperiod of time indicated by the maximum on the graph. For instance,where, as is ordinarily suflicient, it is desired to produce a condenserhaving 1.5 square inches of electrode area per microfarad at 450 voltsworking, 525 volts peak, the foil of the specifications noted whenimmersed for one and one-half minutes in the hydrochloric etching bathof between 2.3 and 2.5 N will present the desired surface gain. Where,however, it is desired to produce foil requiring but 1.0 square inch permicrofarad for a condenser of the same voltage, it would be desirable toimmerse the same in 4.0 N etching solution for a period of four andone-half minutes at 100 degrees F., as best indicated in the graph atthe right of Fig. 2. e

While the presence of aluminum salts in solu tion in the caustic bathhas been found to have no effect upon the efiicacy of cleansing and uponthe gain in effective area attained in the subsequent etching process,the presence of such aluminum salts in the etching bath has been foundto have a decidedly harmful efiect upon the eflicacy of the etching.

In Fig. 3 appears at the left, a series of graphs showing the gain insurface area attained in etching solution of a concentration of 2.5 Ncontaining aluminum chlorid in solution therein at a concentration of1.0 N. It will be'found by comparison with the graph at the left of Fig.2 that the presence of the aluminum chloride up to that concentrationhas practically no effect upon the efficacy of performance temperatureat which such maximum as is attainable under these adverse conditions isreached and that maximum is at a sharp peak, so that control becomesdifiicult in that a small departure from optimum temperature results ina considerable loss in etching eflicacy. In the etching process itistherefore important, not only that the concentration of the etchingacid be kept nearly constant, but also that the concentration of thealuminum chloride dissolved therein be not permitted to exceed 1 N, andpreferably not 0.8 N.

Desirably the etching solution is subjected to a continuous distillationprocess to reclaim the acid for re-use freed of'the aluminum salts. Thisprocess may be conducted according to the pro-.

cedure set forth in the allowed application of Herbert Waterman SerialNo. 191,887 for Art of reclaiming reagent and filed February 23, 1938,on which Patent No. 2,235,658 has since issued on March 18, 1941.

After accounting for and correcting variations in the efficacy ofsurface gain in etched aluminum, that are due to variations in the rawstock, in the concentration of the caustic solution, in theconcentration of the etching solution of hydrochloric acid and theconcentration of aluminum salts therein, further sharp and fortuitousreduction in the surface gain were incurred which for a long timeremained unremedied.

These variations were finally ascribed to the presence of minor amountsof iron salts in the etching solution. As appears from the graph in Fig.4 in which the abscissa isshown on an exponential scale, where theconcentration of iron is as little as .001 gram (1 milligram) per liter,the reduction in the gain of surface area otherwise attainable in theetching process is as much as six per cent. Where, however, the ironconcentration in the etching fluid is one-tenth gram per liter, thereduction in the effective gain of surface area is to well belowone-half of its optimum value. When the iron content in the etchingsolution reaches .5 gram per liter, the reduction in etching gain is soserious as to ren der inadvisable the further use of the solution unlessthe excess iron is first removed therefrom. The graph of Fig. 4 is basedon the use of hydrochloric acid of 2.2 N concentration, in a bath keptat 155 F., the foil being etched for one and onehalf minutes, but thegraph is of substantially the same shape for other concentrations,temperatures and durations of etching.

The technical grades of hydrochloric acid, while sufficiently pure formost chemical processes have been found to contain minor proportions ofiron chloride which vary from lot to lot and which lead to thedifiiculty just set forth. Moreover any small pro ortion of iron on thesurface of the foil that wou.J passoff into the etching solution wouldas it accumulates therein, also lead to the sharp drop from the desiredsurface gain.

That the difliculty was in fact due to the iron salts asset forth, wascorroborated by the 'com plete elimination of the disturbing reductionin surface gain of the etched foil when so-called chemically purehydrochloric acid was used, which has at most minor traces of irontherein.

It'will be understood that while it is preferred to perform the processby operating under the optimum conditions revealed in the graphs in thanthe maximum, by operating as to some or all of these factors at apredetermined value or range below the optimum.

In a typical specification, foil of the purity previously noted and of,the thickness of .00325 inch may in conformity with the graph of Fig.2be etched for one and one-half minutes at a temperature of 155 degreesF. plus or minus 2 degrees in chemically pure hydrochloric acid ofconcentration between 2.3 N and 2.5 N. Equivalent results may beobtained by decreasing the concentration of acid and increasing thetemperature or time of etching, or increasing the concentration anddecreasing the temperature or time of etching in manner clear from thegraphs of Fig. 2. In general, higher concentrations of hydrochloric acidsuch as 6 N or even up to 10 N may be used, but only with thicker foil,because holes would otherwise etch therethrough. With concentrationsabove 10 N the etching eficacy is sharply reduced. Throughout theetching operation the etching liquor should be continuously stirred,desirably by passing compressed air thereinto.

While minor quantities of aluminum salts that pass into the liquor inthe etching operation are not prejudicial, a concentration of aluminumsalts therein in excess of 1 N or preferably of 0.8 N as above pointedout is to be avoided. Tests and adjustments to this end are made,desirably every fifteen minutes and this by a double titration similarto that used for testing and adjusting the caustic solution. Afteretching, the foil is washed with cold tap water, desirably for about 45seconds, to remove the acid. To remove any iron particles that remain onthe surface of the etched foil as well as copper and. any loose aluminumparticles thereon, the foil is then desirably treated in a dilutesolution of nitric acid. A two per cent solution is suitable at 200degrees F. and this treatmentalso may be conducted for 45 seconds. Thenitric acid treatment is especially important when it is desirable toremove the last traces of iron from the surface of the foil, which saidremoval. appears to be desirable for completely satisfactory dielectricfilm formationwhen the foil is to be used as the electrode of adielectric condenser.

The nitric acid treatment appears also to condition the surface forbetter formation character istics as well as to contribute somewhat tothe etching. This appears to be demonstrated by the fact that thecapacitance of condensers prepared from the foil is about five per centlower if the said nitric acid treatment is omitted and better reformingcharacteristics are evident with nitric acid washed foil.

While the plate or foil processed as above set forth may be used forother purposes, the further treatment thereof in a special applicationto electrolytic condensers will now be set forth.

Dielectric film formation in production 'Foil treated and etched asabove set forth is conditioned prior to film formation by treatment inboiling water for a period of preferably not less than one-half minute,although beneficial results are obtained if the treatment in boilingwater is conducted for as little as 10 or seconds or for as much as oneminute or more. T e iling water treatment in some way not understood byme, unlike cold water or even hot water at temperature below the boilingpoint, conditions the surface of the foil to promote the subsequent filmforming procedure. Where the hot water treatment is omitted, in theorder of forty per cent more amperage is needed for formation of thefilm upon the foil for any given voltage, a circumstance which enhancesthe cost not only for power but also for equipment, wiring and the like.

The formation is conducted preferably in a sequence of forming tanks,each containing a suitable electrolyte. Anillustrative film formingtreatment for producing electrodes suitable for condensers of 450 voltworking voltage includes a sequence of three tanks which in a practicalembodiment have the electrolytes and applied voltages as set forth inthe following tabulations:

Tank A. gallons of electrolyte made up of pounds of boric acid inaqueous solutionis kept at a temperature of 190 degrees F. to 210degrees F. and the applied voltage may be 520 volts. Since the initialformation activity in this tank is vigorous and the liquid becomesagitated, it is desirable to cool the contents of the tank byappropriate cooling coils 35 therein which prevent the temperature fromrising excessively.

Tank B.120 gallons of electrolyte made up of 100 pounds of boric acidand 35 grams of borax in aqueous solution is kept at a temperature of190 degrees F. to 210 degrees F, and the applied voltage may be 520volts. Heating coils 36 are provided to bring the temperature to theoperating point, and sometimes to maintain the temperature.

Tank C.In this case the solution of 120 gallons includes 100 pounds ofboric acid and 25 grams of borax and is maintained, at a temperature of190 degrees F. to 210 degrees F. and the applied voltage may be 620volts. The activity in this tank is but low and the same is ordinarilyheated to the desired temperature as for instance by a heating coil 3!to maintain the efficacy of formation.

For production of electrodes suitable for low voltage condensers, thereneed be no graduation of the voltage applied in each forming tank, butthat voltage in every case should be higher than the rated voltage ofthe condenser. For instance for a 110 volt A. C. condenser the formationis de-' sirably 180 volts in each tank.

The solutions in the tanks are tested and readjusted from time to timeto maintain con stancy of operation.

After the condenser sections have been built according to conventionalpractice from foil with both electrodes, or only. the anode in the caseof direct current condensers, made of the processed foil as abovedescribed, the sections are aged by applying thereto desirably a seriesof progressively higher voltages. For a 450 volt working voltagecondenser for instance, the voltage may be applied in four steps of 200,300, 400 and 500 volts, the last sa for one hour. Thereupon the sectionsare tempered in an oven at about degrees F. for from two to four hours.The latter operation enables the units to withstand higher temperaturein use and brings about a lower leakage and faster formation at roomtemperature.

By the process set forth, a gain in capacity per unit of area isattained in the order of 400 to 1000 per cent as contrasted with foilprepared without the etching and the various procedural steps thecompleted condenser of only. ten per centfrom rated reading. Accordinglythe present rocess eliminates the need for the great excess of foillength required in the case of foil treating processes that lead to-muchwider variations in capacity of the completed condenser.

In the diagram of Fig. 5, is shown an installation for pre-treating andetching the aluminum foil for condenser electrode or other use.

Foil from the spool F is passed through tank In which is charged withthe caustic solution- H. The foil after it leaves the solution is washedby the cold water spray delivered through pipes l2. The foil then passesinto the tank l3 containing the hydrochloric acid etching solution Hwhich is maintained stirred or agitated by compressed air admittedthrough pipe I5. I Thence the foil passes a spray of water deliveredthrough pipes 16 and passes into and through a tank I! charged at ISwith the dilute nitric acid. The nitric acid is washed off the foil by aspray of Water from pipe i9, whence the foil passes through an electricdryer 20 and the clean etched foil is'rolled upon reel 2|.

In the preparation of electrodes for electrolytic condensers, thecaustic and etching baths would be controlled as previously set forth.In an advantageous commercial application, each area of foil remainsimmersed in the caustic bath II for about one-half minute. It is washedwith water at l2 for about ten seconds before entering the etching bathl4, in which latter it remains for about 1 /2 minutes. The foil iswashed for about 45 seconds at i6 after leaving the etching bath and issubmerged for 45 seconds in the nitric acid bath 18, the final waterwash at I! is also about seconds, and the traverse through the dryingheater takes 20 seconds,

In the diagram of Fig. 6 is shown an installation for further treatmentof the foil to produce formed electrodes. The foil from reel 2| ispassed through a tank 22 charged at 23 with water maintained at boilingtemperature by coil 24. Thence the foil is fed in succession through thethree forming tanks A, B and 0 charged with the electrolytes previouslydescribed. Current is passed to the tanks at the respective voltages bymotor generators 26, 21 and 28 respectively, the circuit to the foilbeing completed at metal roller 29. Distilled water for washing the foilafter it leaves the tank C is shown passed through pipe 30. The washedfoil is dried by electric dryer 3| and collected upon roll 32.

Motor starting condensers In preparing the foil, the causticpre-treating operation and the etching operation may be the same asthose previously described but the forming operation is modified.

The formation in the first tank A is conducted in an electrolytecomposed of an aqueous solution with a low concentration of boraxtherein. desirably three pounds of borax in 100 pounds of watermaintained at about 200 degrees F. during formation. Borax in theinitial tank is greatly superior to boric acid for the production ofcondensers of low power factor. A high concentration of borax in theinitial tank has however been found to attack the foil and reduce theetching ratio.

In the second and third tanks (B andC), 120 gallons of electrolytecontains about 750 grams of borax and pounds of boric acid.

factor of the motor starting condensers, the pretreatment in boilingwater is omitted and the forming electrolyte should be frequentlyreplaced,'because substantial quantities of aluminum oxide and hydroxidein the electrolyte impair the power factor. After the formation, thefoil is washed as previously described.

As many changes could be made in the above method and many apparentlywidely different embodiments of this invention could be made withoutdeparting-from the scope of the claims, I

it is intended that all matter contained in the above description orshown in the accompany: ing drawings shall be interpreted asillustrative and not in a limiting sense.

Having thus described my invention, what I claim as new anddesire tosecure by Letters Patent is: v

1. The processing of aluminum sheet material to attain a substantiallyuniformly large gain in surface area thereof, which consists inpretreating the same in a caustic bath, while maintaining theconcentration and temperature of the latter within a limited range ofvariation and then treating the sheet material for a predeterminedperiod of time in an etching acid bath maintained within a limited rangeof variation of concentration and ate. predetermined ternperature thataffords substantially the optimum of etching eflicacy for saidconditions as indicated by the maximum point on each of the family ofcurves of Fig. 2 of the drawings.

2. The processing of aluminum sheet material to attain a substantiallyuniformly large gain in surface area thereof, which consists inpretreating the same in caustic solution maintained at a temperature ofapproximately 200 degrees F. and of concentration within a limited rangeof variation and after washing-the foil treating the same for apredetermined period of time in an etching bath of hydrochloric acidmaintained within a limited range of variation in concentration and at apredetermined temperature that affords substantially the optimum-ofetching efficacy for said conditions as indicated by the maximum at eachof the family of curves of Fig. 2 of the drawings.

3. The processing of clean aluminum sheet material in the order of 99.8per cent pure to attain a substantially uniform gain in surface areathereof, which consists in treating the same for a predetermined periodof time in a bath of hydrochloric acid solution within a limited rangeof variation in concentration and at a temperature that afiordssubstantially the optimum of etching efficacy for said conditions asindicated by the maximum at each of the family of curves of Fig. 2 ofthe drawings.

4. The processing of aluminum sheet material in the order of 99.82 to99.85 per cent pure to attain a substantially uniform high gain insurface area thereof, which consists in treating the same in a bath ofsodium hydroxide of approximately two per cent concentration and at atemperature of approximately 200 degrees'F.

for about one-half minute and then treating the foil in a bath ofhydrochloric acid solution of 2.3 to 2.5 N at a temperature of aboutdegrees F. for a period of about one and one-half minutes.

5. The processing of aluminum foil for a substantially predeterminedgain in surface area, which consists in passing the foil through acaustic solution held to close limits of concen- In order to maintainthe desired low power.

tration and at a temperature in the order of 200 degrees F. and thentreating the foil in a solution of hydrochloric acid at substantially agiven concentration and temperature, while controlling the etchingliquid to maintain below 1 N the concentration of aluminum saltsdissolved therein.

6. In the process of etching aluminum foil for substantially apredetermined'surface gain, the

' material for substantially a predetermined surface gain, the step ofpassing the material through a bath of hydrochloric acid maintainedsubstantially, at constant concentration below N, and at substantiallyconstant temperature and with the ironimpurity content of said etchingbath maintained at all times at less than .005 gram per liter, andhaving aluminum salt dissolved therein not exceeding a concentration of1 N.

8. The processing of aluminum sheet material for substantially constantgain in surface area, which consists in pre-treating the material in abath of sodium hydroxide, maintaining said bath at substantiallyconstant concentration and temperature, then passing the materialthrough a bath of hydrochloric acid solution maintained at nearlyconstant temperature below 6 N while maintaining the iron impuritycontent of said etching bath below .005 gram per liter.

9. The processing of aluminum sheet material for substantially constantgain in surface area, which consists in pre-treating the material in abath of sodium hydroxide, maintaining said bath at substantiallyconstant concentration and temperature, then passing the materialthrough a bath of hydrochloric acid solution at concentration below 6 N,maintained at nearly constant temperature, while maintaining the ironim= purity content of said etching bath below not gram per liter, withthe aluminum salt concentration not exceeding 1 N.

10. The processing of aluminum sheet material 99.82- to 99.85 per centpure with iron con= tent of between .08 and .12 per cent and siliconcontent of about .06 per cent, and of the degree of hardness attained byannealing the same for two days at 900 degrees F. followed by a slowcooling, which consists in passing the same through a bath of sodiumhydroxide or concentration of substantially two per cent and temperatureof approximately 200 degrees F., washing the foil and then passing itthrough a bath of hydrochloric acid substantially devoid of iron,maintained at a substantially constant concen= tration in the order of 2to 4 N, and at a sub till stantially fixed temperature, whilemaintaining the concentration of aluminum salts in the etching bath at avalue not exceeding 1 N.

11. The process which consists of the steps of etching clean aluminumsheet material in a, solution of hydrochloric acid, washing the samewith cold tap water for less than one minute to remove acid, andremoving any iron, copper and loose particles of aluminum andconditioning the surface for better film formation characteristicsby'treatment in a two per cent solution of nitric acid at about 200degrees F. for less than one minute.

12. The processing of clean aluminum foil in preparation for filmformation of electrolytic condensers, which consists in etching the foilin a dilute hydrochloric acid bath, while agitating the bath andremoving therefrom aluminum salts in excess of a concentration of abouti N, and then treating the foil. with. boiling water for about one-halfminute.

13. The processing of clean aluminum foil in preparation for filmformation of electrolytic condenser, which consists in etching the foilin hydrochloric acid of high purity and substantially devoid of ironcontent and in concentration of between 2.3 and 2.5 N at a temperatureof about degrees F. and for a period of about one and one-half minutes,washing the etched foil with cold water and, thereupon washing it with adilute solution of nitric acid followed by treatment with boiling waterfor a period of between ten seconds and one minute.

14. The processing of clean aluminum foil approximately 99.8% pure inpreparation for film formation of electrolytic condensers which consistsin etching the same in a dilute bath of hydrochloric acid substantiallydevoid of iron, washing the etched foil with cold water, thereuponwashing it with a two per cent solution of nitric acid at 200 degrees F.and then rinsing the film with water and promptly drying the same, andconditioning the film by treatment in boiling water for a period notless than one-half minute.

15. The processing of clean aluminum foil that is at least 99.8% pure inpreparation for film formation of electrolytic condensers which consistsin first treating the surface of the foil with caustic at controlledtemperature and concentration, washing the foil thus treated, etchingthe clean foil in a bath of dilute hydrochloric acid substantially freefrom iron, removingacid from the etched foil by washing in water,removing any impurities of iron, copper or loose particles of aluminumand conditioning the surface for better film formation characteristicsby washing in dilute nitric acid, rinsing off the nitric acid with waterand promptly drying the foil and thereupon treating the dried foil inboiling water for a period of about one-half minute.

GEORGE FREDERICK COGGINS.

